Beef And Onion Stew

• 1 bay leaf
• 1 sprig thyme (fresh)
• 1/2 lemon
• 450 gr leftover beef
• 55 gr butter
• 16 ml dijon-type mustard
• 9 gr parsley (chopped fresh)
• 3 slices onions (yellow)
• 1 slice from pot-au-feu
• some stock